The invention relates to nanostructured titania coatings and more particularly to nanostructured titania-coated titanium balls and seats in ball valves designed for handling very corrosive and abrasive fluids.
Ball valves and other components used in the pressure acid leach process, and especially the nickel-cobalt high pressure acid leach (NiHPAL) process, are subject to a severely corrosive environment of very strong acids and high pressure and temperature. Valve costs typically represent just 1%-2% of the total plant costs; however, their influence on productivity can be much greater. For example, maintenance costs for NiHPAL service are six times greater than originally expected; valve maintenance represents 30%-40% of the total expense.
Conditions that create the severe service in the NiHPAL process include high temperatures, abrasive solids, and acid corrosion. Temperatures are typically 260xc2x0 C., and process engineers desire to raise this to 288xc2x0 C., or even 316xc2x0 C. These temperatures prevent the use of soft sealing materials such as TEFLON or PEEK. The elevated temperatures magnify the corrosivity of the lading and introduce thermal stresses at coating-substrate interfaces. Crushed solids transported through the system abrade and damage the precision sealing surfaces of the valve seats. Solids tend to pack into clearances within the valve internals and prevent proper float of the sealing members, resulting in seat leakage and rapid erosive wear. Corrosion due to the acidic environment can attack and deteriorate precision sealing surfaces, develop corrosion products at the valve coating-substrate interface, and result in spalling of the coating. Corrosion products can also expand to fill tight clearances in seats, again preventing proper float, resulting in seat leakage and rapid erosive wear. Corrosion can cause deterioration of the integrity and wear resistance of the coating.
The limits in the overall performance of severe-service ball valves are constantly being challenged by more arduous operating conditions. In many instances, the exposure of the components to aggressive wear in extremely corrosive environments necessitates a compromise in mechanical integrity to attain sufficient chemical stability. The goal is to reduce or eliminate the compromise in mechanical integrity while maintaining chemical stability.
Severe-service ball valves such as in NiHPAL commonly incorporate coatings to enhance their reliability and life against extreme wear and corrosion. Because of the severe conditions, efforts to reduce the failure rate have included, for example, the use of valve components made from titanium coated with microstructured (grain or particle size in micron range) plasma-sprayed chromium oxide. In many industrial processes, maintenance and downtime associated to valve wear and failure can be substantial; hence, enhancement in the ball valve reliability and life may play an important role in operating costs. Until now, most of the focus on thermal spray coatings of ball valve components has revolved around the composition of microstructured coatings and the method of application. By modifying the microstructure, one can greatly enhance the mechanical properties of the coatings with little or no change to the chemical properties. Although there remain newer coating compositions and deposition methods to be tried, most of the obvious options have been studied.
A particular problem with coating titanium substrates in NiHPAL service is that the coating often has a different coefficient of thermal expansion (CTE) that results in residual stresses due to different rates of thermal expansion and contraction upon heating and cooling, respectively. The presence of excessive residual stress can result in premature spalling (debonding) of the coating from the substrate and/or higher cross-sections of crack formation within the coating. A metallic bond coat has been used to reduce the CTE mismatch between the metallic substrate and the ceramic coating, as well as to provide a physical barrier against substrate corrosion. To date, however, no metallic bond coat layer has been successful to improve performance of the ball valves; this is due to the very high pressures and severe corrosivity of the slurry used in NiHPAL processes, which attacks the bond coat layer and the substrate.
In recent years, much interest in the field of materials science has been focused on the area of ultrafine-grained or nanostructured materials. Nanostructured materials possess a physical feature (e.g., grain size, particle reinforcement) that is less than 100 nm. As used in the present specification and claims, xe2x80x9cultrafinexe2x80x9d refers to materials having a physical feature less than 300 nm. These materials have unique properties such as enhanced hardness, wear-resistance, and strength for metals; enhanced toughness and reduced sintering temperature for ceramics; and enhanced wear-resistance and toughness for ceramic-metal composites. Even more recently, there has been a strong effort towards incorporating these enhanced properties onto surfaces of components by way of thermal spray coatings. Representative patents directed to thermal spraying and coatings include, for example, U.S. Pat. No. 5,874,134 to Rao et al.; U.S. Pat. No. 5,939,146 to Lavernia; and U.S. Pat. No. 6,025,034 to Strutt et al.; each of which is hereby incorporated herein by reference.
A critical requirement in any thermal spray process is attaining a starting powder with a certain size range and composition. For ultrafine-grained coatings, one approach is in using agglomerated ultrafine-grained powder of the required size and composition. An agglomerated powder is required since the ultrafine particles are much too fine to be fed through conventional powder feeders; the fine powders would also vaporize upon exposure to the hot section of the jet or flame.
Thermal spray processes are commonly used to heat and accelerate the feed particles onto a substrate, thereby forming a coating. Due to the high thermal energy required to sufficiently soften or melt ceramic particles, not all thermal spray systems are feasible for depositing ceramic coatings.
The present invention is directed to nanostructured titania coatings that can be prepared by thermal spray coating ultrafine titania agglomerates onto a titanium substrate surface. The abovementioned and other deficiencies of the prior art are overcome or alleviated by the methods of the present invention, which will enhance the reliability and the life of ball valves by incorporating superior coatings with ultrafine-grain size.
In one aspect, the present invention provides spherical agglomerates useful in thermal spray coating. The agglomerates have a size range of from 5 to 100 microns, preferably 10 to 45 microns, and comprise a mixture of ultrafine titania particles of less than 0.3 microns, and from 5 to 45 volume percent, by total volume of the particles, of ultrafine particles selected from the group consisting of zirconia, tantalum oxide, boron carbide, silicon carbide, titanium carbide, diamond and combinations thereof.
In another aspect the present invention provides an ultrafine, preferably nanostructured titania coating bonded directly on a substrate of titanium. The coating can have a thickness of up to 500 microns, or be ground and polished to 100 to 200 microns. The coating includes a grain growth-inhibiting proportion of a second phase material immiscible with the titania. Preferably, the coating includes from 5 to 45 volume percent of a material selected from the group consisting of zirconia, tantalum oxide, boron carbide, silicon carbide and combinations thereof. In a preferred embodiment, the coating has a ground and polished surface.
A further aspect of the invention is the provision of a method for applying an ultrafine, preferably nanostructured titiania coating. The method includes the steps of: (a) preparing agglomerates comprising a mixture of ultrafine titania particles and ultrafine second-phase particles that are immiscible with titania and corrosion resistant; (b) thermally spraying the agglomerates onto a substrate surface to deposit a coating of ultrafine titania thereon; and (c) optionally grinding and polishing the coating. The substrate is preferably titanium. The mixture can include from 5 to 45 volume percent, by total volume of the particles, of ultrafine second-phase particles selected from the group consisting of zirconia, tantalum oxide, boron carbide, silicon carbide, titanium carbide, diamond and combinations thereof.
Yet another aspect of the invention is the provision of a ball valve for handling very corrosive fluids and abrasive solid particles in a pressure leaching process. The ball valve includes a valve body, a ball centrally positioned in the valve body and having a central passage rotatable in the valve body between open and closed positions, and at least one seat disposed between the ball and the valve body. The ball and seat each comprise a titanium substrate and an ultrafine, preferably nanostructured titania coating. The coating can have a titania phase and a phase immiscible with the titania phase in a proportion effective to inhibit grain growth. The immiscible phase preferably comprises from 5 to 45 percent by volume of the coating. The immiscible phase can be selected from zirconia, tantalum oxide, boron carbide, silicon carbide, titanium carbide, diamond and combinations thereof. The coating can have a ground and polished surface. The coating can have a thickness from 100 to 500 microns, or preferably when it has a ground and polished surface, a thickness of from 100 to 200 microns. The titania coating preferably has a grain size less than 500 nm. The coating is preferably deposited by thermal spray application of a powder comprising spherical agglomerates in a size range of from 10 to 45 microns comprising a mixture of ultrafine particles of less than 0.3 microns.
A still further aspect of the invention is a pressure acid leaching process comprising alternately opening the ball valve just described to allow passage of an acid leach mixture comprising abrasive particles and closing the ball valve to stop said passage, wherein the ball and seat are protected from wear by the titania coating.
A further aspect is a system for applying an ultrafine, preferably nanostructured titiania coating. The system includes means for preparing agglomerates comprising a mixture of ultrafine titania particles, a reservoir comprising a charge of the titania agglomerates, and means for thermally spraying the agglomerates from the reservoir onto a substrate surface to deposit a coating of ultrafine titania thereon.